@ Yi Wang: Thank you for your comment.
I checked in the power-user-preference list, where for peer_expiration_days is listed: "Number of days to pass before a peer is removed from the peer list".
Out of that explanation, it is not clear to me, which kind of property a peer has to have to actually get removed from the peer list, since the peers which are offline and disconnected are listed in the Peer list for way over 7 days.